Cube Calculations To Date behaviour

The cube developed is for an origanization that manages and conducts huge projects.
All project calculations are based on the life of a project, in other words beginning of time to date.

In other words:

Revenue Recognition to Date = Actual Cost to Date / Budget Cost to Date * Budget Revenue To Date

Revenue Recognition for Current Period =
([Time].[Year - Month].CurrentMember,[Measures].[Revenue Recognition to Date])-
([Year - Month].PrevMember,[Measures].[Revenue Recognition to Date])

This gives current period accurately, but when a user selects inidividual periods, say January 2012, March 2012, April 2012, the sum (Grand Total) of the report does not properly add up. It still reflects the total as the "To Date" value.

Any ideas how this behaviour can be changed?

How have you defined [Measures].[Revenue Recognition to Date] ?

I actually show it in the post:

Revenue Recognition to Date = Actual Cost to Date / Budget Cost to Date * Budget Revenue To Date

mdx = [Measures].[Actual Cost to Date /([Measures].[Budget Cost to Date] * [Measures].[Budget Revenue To Date].

the problem is not the accuracy of the calculation or even the inidividual months, it is how it behaves when only certain months are selected.

If if every month is shown, it shows an accurate grand total, if only some months are selected, it still shows the same grand total as that of all the months added together.